APOLLO, on the Omphalos

Apollo, seated on the omphalos, holding an arrow,
his bow resting on the omphalos , the sacred stone
placed at Delphi by Zeus.

When Zeus was born, to prevent him from being swallowed by Kronos, Rhea and Gaia substitued a stone wrapped in swaddling clothes, which Kronos indeed swallowed. When he was forced to regurgitate it later, along with Zeus' five siblings, the stone became a relic of the occasion. See Hesiod, Theogony 467-500.